Processes of producing lithographic electrostatic printing plates



Oct. 15, 1963 D M. BORNARTH 3,107,169

PROCESSES OF PRODUCING LITHOGRAPHIC ELECTROSTATIC PRINTING PLATES Filed Dec. 18, 1958 Y 25/9070 Reap ucr FUSED 7011/5 2 4 United States Patent Ofi ice 3,107,169 Patented Get. 1 5, l 963 [mil 3,167,169 PROCESSES F PRGDUCHNG LITHUGRAPHTC ELECTROSTATIC PRINTING PLATES Dennis M. Bernarth, Rolling Meadows, IiL, assignor to Charles Bruning Company, The, Mount Prospect, 111.,

a corporation of Delaware Filed Dec. 18, 1958, Ser. No. 781,352 Claims. (Cl. 961) This invention relates to the production of lithographic printing plates and to such plates having a hydrophilic surface capable of receiving and retaining, during the printing operation, an aqueous wet-out solution immiscible with greasy lithographic printing ink and a hydrophobic surface capable of receiving the greasy lithographic printing The ink is transferred from this hydrophobic surface directly, or indirectly through a conventional blanket, to paper or other printing surfaces.

This invention is particularly applicable to the production of lithographic printing plates having a paper base by treating a paper base having a photo-conductive insulating surface by an electrostatic printing technique to produce a visible record or copy, hereinafter referred to as an electrostatic copy, and conditioning this electrostatic copy to produce a lithographic printing plate which can be employed to make any desired number of additional copies.

Electrostatic printing techniques are known in which a surface of a conductive or semi-conductive backing or substrate having a photo-conductive insulating material thereon is charged electrostatically, and thereafter exposed to an optical image produced thereon by light rays which effect the discharge of the portions irradiated by the light rays while leaving the remainder of the surface in a charged condition, thus forming a latent electrostatic image. Toner or developer powder is applied to the electrostatic latent image and held thereon by electrostatic attraction, thus producing a powder image which is fixed preferably by fusion, but may be fixed by use of solvent causing the powder particles to coalesce, or other known procedures. An electrostatic copy of the original results.

It is among the objects of the present invention to provide a process of producing a lithographic printing plate from such electrostatic copy quickly and efiiciently. By quickly is meant of the order of a few minutes or less. In other words, the present invention permits conditioning an electrostatic copy by a simple operation requiring no special skill to produce a lithographic printing plate in a matter of about one minute, and sometimes less, which lithographic plate can be used to make additional copies, for example, in a conventional offset press. Thus the interval between completion of the electrostatic copy and its use on a press to make additional copies may be astonishingly short.

Another object of this invention is to provide a method of converting such electrostatic copy to a lithographic plate, which method is simple and fast, involving as it does a one step treatment of the copy, and results in a master or lithographic printing plate which can be run immediately to produce additional copies.

Other objects and advantages of this invention will be apparent from the following detailed description thereof.

The lithographic printing plate produced by the process of this invention comprises a substrate or support, which preferably is paper including high Wet strength paper, an electrically-insulating resinous film-forming vehicle on this substrate having imbedded therein photo-conductive zinc oxide, a layer of hydrophobic resin covering the image areas, and a hydrophilic film formed by the reaction of a halide or sulphate of aluminum or iron (ferric state) with zinc oxide covering the non-image or background areas.

Such lithographic printing plates can be made by charging the zinc oxide containing photo-conductive surface on the substrate electrostatically, exposing the charged member to produce thereon a latent image of the material to be copied, applying resinous toner or developer powder so that it adheres electrostatically to the image areas and not to the non-image or background areas, fusing or otherwise fixing the resinous toner and then swabbing or wiping the resultant product with an aqueous solution of a halide or sulphate of aluminum or ferric iron. Surprisingly, I have found that a reaction takes place substantially instantaneously between the aluminum or iron salt and the zinc oxide irnbedded in this resinous vehicle in the non-image areas Where the Zinc oxide is not covered by the fixed resin derived from the toner or developer particles, forming a film which has hydrophilic properties and which resists penetration by the fountain solution, whereas the aluminum or iron salt solution thus applied does not deleteriously aflect the hydrophobic properties of the resin defining the image areas. This reaction takes place so quickly that in a matter of a minute or less after application of the aqueous solution of aluminum or iron salt, the electrostatic copy is converted to a lithographic printing plate which can be used to make additional copies.

In practice, it has been found that it is only necessary to take the electrostatic copy as produced, swab it with the aluminum or ferric salt solution once, attach the resulting printing plate to the cylinder of an offset printing machine and proceed to run the machine by applying fountain solution and ink as conventional. Thus the time elaspsed between production of the electrostatic copy and its utilization as a lithographic printing plate in accordance with this invention may be of the order of a minute or less.

Should the printing plate produced in accordance with this invention be permitted to stand for say 10 to 15 minutes or longer, the hydrophilic properties of the film in the non-image area may deteriorate somewhat. This film can be restored by giving the resultant plate an addi tional treatment with the aqueous solution of aluminum or ferric salt similar to the initial treatment.

The accompanying drawing. shows for purposes of exemplification, a cross-section through a lithographic printing plate produced by the process of the present invention. The drawing shows the parts on an enlarged scale to better illustrate the lithographic printing plate.

In the drawing, 1 indicates the substrate or base which preferably is paper, say a high wet strength paper, but may be a plastic film, metal foil, or any other base which is electrically-conductive or semi-conductive. This base 1 is coated with an electrically-insulating resinous filmforming vehicle 2 having imbedded therein a photo-conductive grade of zinc oxide in particulate form, i.e., particles 3 of zinc oxide having a size of the order of. .2 micron. An example of such zinc oxide is the Florence Green Seal 8 photo-conductive zinc oxide sold by the New Jersey Zinc Company. It will be understood that any photo-conductive zinc oxide in particulate or finely divided form may be employed, and this invention is not to be limited to either the particular size or grade of zinc oxide hereinabove disclosed for purposes of exemplification.

As the resinous vehicle in which the zinc oxide is suspended and as thus suspended applied to the substrate 1 to form the resin layer 2, any electrically-insulating filmforming resin may be used. Examples of such resins are the Well known silicone resins, e.g., G.E. SR-82 marketed by the General Electric Company; melamine-formaldehyde resins, e. g., those marketed by the American Cyanarnid Company; polyvinyl acetate resins, e. g., Elvacet marketed by Du Pont; alkyd resins, e.'g., Synthemul; and mixtures of melamine-formaldehyde resins with thermoplastic resins such as the vinyl acetates, vinylidene chloride, acrylonit'rile-butadiene copolymers (Hycars), styrenebutadiene copolymers (Latex 512R), copolymers of styrene with acrylonitrile, acrylates and/or methacrylic acid, such as the resin sold by Monsanto under its Lytron trademark and which is disclosed in United States Patent The resinous vehicle containing the photo-conductive zinc oxide may be applied to the substrate 1 by dissolving the resin in a suitable solvent, adding the zinc oxide to form a suspension of the zinc oxide in the resultant resin solution and applying the resultant suspension to the substrate 1. Alternatively an emulsion of the resin or resin mixture may be produced, the zinc oxide suspended in the emulsion and this emulsion system applied to the substrate 1 to form the photo-conductive insulating layer 2 containing particles 3 of photo-conductive zinc oxide imbedded in the surface thereof.

In accordance with well known electrostatic copying techniques, the resultant base is charged, for example, by exposing it to corona discharge or by passage over a charging roller which applies a uniform electrostatic charge over the entire surface thereof. The charged member is then exposed to produce a latent image of the material to be copied. Developer powder or toner is then applied to the image areas and held thereon by electrostatic attraction; the toner is thereafter fixed, i.e., made relatively permanent by heat fusion or solvent solution followed by evaporation of the solvent.

The toner or developer powder is a resinous material which when fixed has hydrophobic properties and will attract greasy inks. Examples of such developer powders are pigmented styrene polymers or polymers of substituted styrenes, e.g., Piccolastics marketed by the Pennsylvania Industrial Chemical Corporation of Clairton, Pennsylvania, pigmented phenol formaldehyde resins and other resins having hydrophobic properties.

The developer powder may be applied directly to the latent image or admixed with a carrier such as glass beads. Preferably it is applied in the form of a mixture with magnetic particles such as magnetic iron, which mixture results in imparting a charge to the developer powder particles triboelectrically. The developer powder is so chosen that it is attracted electrostatically to the chargedimage or repelled from the background area to to the charged image and held thereon by electrostatic attraction. It is preferred to apply a negative charge to the photo-conductive insulating material. A positive toner is applied when the image areas have a negative charge. When the image areas have been discharged relative to the background areas, a negative toner is applied.

A preferred negative toner, to which however the invention is not limited, is a mixture of a resin polymer of styrene and styrene homologs (Piccolastic D), a similar resin modified by reaction with an alkyl ester of an aryl acid (Piccolast-ic C), carbon black and other pigment. This negative toner is produced by mixing the resinous constituents with the carbon black and/or other dyestufi, melting the resins, solidifying the molten mixture, grinding the solid and mixing the ground particles with magnetic material such as iron.

An exemplary positive toner useful in producing the printing plates of this invention may consist of a mixture of Piccolastic D, Piccolastic C, polymerized butene resin, carbon black, Nigrosine dye and Iosol black. These constituents are mixed, the resins melted, the molten mixture solidified, ground to fine particle size, and the solidiiied ground particles mixed with magnetic iron imparting to the resin particles a positive charge triboelectrically.

Fixing of the toner particles on the image areas results in a layer 4 of hydrophobic resin covering the zinc oxide 4 in the image areas. In the preferred embodiment, layer 4 is a layer of fused resin produced by subjecting the image covered with the toner particles to heat to fuse the toner particles in the image areas.

The electrostatic copy produced by fixing the toner particles held electrostatically on the image areas is swabbed with an aqueous wet-out solution of an aluminum or ferric halide or sulphate. While aluminum chloride solutions are preferred, aqueous solutions of ferric chloride, aluminum or ferric bromide, aluminum iodide, or aluminum or ferric sulphate can be used. The concentration of the solution may be from 1% to 40% by weight of the aluminum or ferric salt, preferably 2% to 1 5% by weight. The pH of the solution should be below 6, preferably from 1 to 3.5.

The welt-out solution may contain other constituents, such, for example, as glycerin, which is added primarily because it is hygroscopic and hence by absorption of water prolongs the period during which the hydrophilic surface 5 produced retains its hydrophilic properties. In other words, by the application of wet-out solutions containing glycerin, the resultant lithographic printing plate can be kept before use for relatively longer periods of time as compared with the use of a wet-out solution containing only the aluminum or ferric salt. Other constituents which may be present are ammonium sulphate, gum arabic, formaldehyde and any of the constituents normally used in standard wet-out solutions, such, for example, as phosphoric acid. Solutions containing formaldehyde and gum arabic generally result in lithographic printing plates which, after removal from the press, retain the hydrophilic properties in the non-image areas for relatively long periods of time so that the resultant printing plates can be reused without subjecting them to an addition& treatment with the wet-out solution.

The following examples of wet-out solutions which may Wet-Out Solution #1 4 parts aluminum chloride 96 parts water Wet-Out Solution #2 4 pants of aluminum chloride 6 parts glycerin parts water Wet-Out Solution #3 4 parts aluminum chloride 1 part ammonium sulphate 6 parts glycerin 111 parts water Wet-Out Solution #4 5 parts aluminum sulphate 1 part aluminum chloride 6 parts glycerin 1'11 parts water Wet-Out Solution #5 4 parts aluminum bromide 6 parts glycerin 90 parts water Wet-Out Solution #6 5 parts aluminum iodide 6 parts glycerin 90 pants water Wet-Out Solution #7 5 parts aluminum sulphate 6 parts glycerin 111 parts water Wet-Out Solution #8 4 parts aluminum chloride 1 part magnesium nitrate 1 part formaldehyde 60 parts gurn arabic'(10% gum) 30 parts water Wet-Out Solution 4 parts aluminum chloride 2 parts phosphoric acid 50 parts gum arabic'(10% gum) 1 part formaldehyde 30 parts water Wet-Out Solution 5 parts aluminum chloride 95 parts of a commercial wet-out solution containing as its principal constituents glycerin and phosphoric acid Wet-Out Solution #11 4 parts ferric chloride 96 parts water Wet-Out Solution #12 4 parts of ferric chloride 6 parts glycerin 90 parts water Wet-Out Solution #13 4 parts ferric chloride 1 part ammonium sulphate 6 parts glycerin 111 parts water Wet-Out Solution #14 5 parts ferric sulphate 1 part ferric chloride 6 parts glycerin 111 parts water Wet-Out Solution #15 4 parts ferric bromide 6 parts glycerin 90 parts water Wet-Out Solution #16 5 parts ferric sulphate 6 parts glycerin 1 11 parts Water Wet-Out Solution #17 4 parts ferric chloride 1 part magnesium nitrate 1 part formaldehyde 60 parts gum arabic'(10% gum) 30 parts Water Wet-Out Solution #18 4 parts ferric chloride 2 parts phosphoric acid 50 parts gum arabic'(l0% gum) 1 part formaldehyde 30 parts water Wet-Out Solution #19 5 parts ferric chloride 95 parts of a commercial wet-out solution containing as its principal constituents glycerin and phosphoric acid The above Wet-out solutions are stable on storage. They can be stored indefinitely without decomposition. When applied to the electrostatic copy, for example by swabbing the surface, each application requires only a few seconds. Surprisingly and unexpectedly, the aluminum or ferric halide or sulphate solution reacts with the zinc oxide 3 (which zinc oxide is exposed in the nonimage areas) to produce a hydrophilic surface 5 which prevents the penetration of the fountain solution in these areas and repels inks employed in lithographic printing. No such reaction takes place in the image areas, in which the zinc oxide is covered by the resinous toner layer 4, but the resinous coating 4 in the image areas retains its 6 hydrophobic properties and receives the greasy ink for transfer to the printing surface.

A lithographic printing plate produced in this manner can he run immediately or may he kept for as long as 10 minutes without re-etching, i.e., reprocessing with the Wet-out solution before being run. In the case of the wetout solutions containing glycerin or other hygroscopic constituent, the plate after treatment may be kept for a somewhat longer period before requiring reprocessing with the wet-out solution.

It will be noted that the present invention provides a process of producing lithographic printing plates from electrostatic copies, quickly and efficiently. It will be further noted that the method of this invention involves a one step treatment of the electrostatic copy and results in a master or lithographic printing plate which can be run immediately to produce additional copies. This invention has been successfully practiced with a time interval of less than one minute, of the order of 40 to 50 seconds, :between the completion of the electrostatic copy and the utilization of the lithographic printing plate made therefrom to produce additional copies.

Since certain changes in carrying out the process which embody this invention may be made without departing from the scope of this invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of producing lithographic printing plates, which comprises, charging a support having a photo-conductive surface consisting essentially of photo-conductive zinc oxide imbedded in an electrically-insulating resinous film-forming layer covering the face of said support to produce an electrostatic charge thereon, exposing the charged support to a light image of material to be copied to produce a latent electrostatic image of said material, applying hydrophobic resinous developer powder to the latent electrostatic image thus produced so that said bydrophobic resinous powder adheres electrostatically to the charged image and not to the back-ground areas of said photo-conductive surface, fixing the hydrophobic resinous powder to produce an electrostatic copy in which the hydrophobic resinous layer thus formed defines the image areas and the said electrically-insulating resinous layer having the zinc oxide imbedded therein constitutes the background areas, and swab'hing the thus produced electrostatic copy with an aqueous solution of a salt from the group consisting of the halides and sulfates of aluminum and ferric iron to react the zinc oxide in the said background areas with the said salt and thus produce a solid hydrophilic film constituted substantially entirely of the reaction product of said salt and the zinc oxide in the background areas, which film covers the background areas, resists the penetration of lithographic fountain solutions and repels lithographic inks.

2. The method of producing a lithographic printing plate as defined in claim 1, in which said aqueous solution has a pH below 6 and contains from 1% to 40% by weight of said salt.

3. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous solution applied to the electrostatic copy contains a hygroscopic material in addition to said salt.

4. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous swabbing solution is an acidic solution of aluminum chloride containing from 1% to 40% by weight of aluminum chloride.

5. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous swabbing solution is an acidic solution of aluminum chloride containing from 2% to 15% by Weight of aluminum chloride.

6. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous sw-abbing 7 solution is an acidic solution of ferric chloride containing from 1% to 40% by Weight of ferric chloride.

7. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous s-Wabbin-g solution is an acidic solution of ferric chloride containing from 2% to 15% by Weight of ferric chloride.

8. The method of producing a lithographic printing plate as defined in claim 3, in which the hygroscopic material is glycerin.

9. The method of producing a lithographic printing plate as defined in claim 4, in which the acidic constituent of said solution of aluminum chloride is phosphoric acid.

10. The method of producing a lithographic printing plate as defined in claim 1, in which the aqueous solution of said salt from the group consisting of the halides and sulfates of aluminum and ferric iron is an acidic solution having a pH below 6 and containing phosphoric acid as the acidic constituent.

References Cited in the file of this patent UNITED STATES PATENTS 2,760,431 Beatty Aug. 28, 1956 2,857,271 Sugarman Oct. 21, 1958 2,857,272 Greig Oct. 21, 1958 2,957,765 Resetich Oct. 25, 1960 2,983,220 Dalton et al May 9, 1961 2,993,787 Sugarman July 25, 1961 3,001,872 Kurz Sept. 26, 1961 FOREIGN PATENTS 203,907 Australia Nov. 1, 1956 633,747 Great Britain Ian. 28, 1953 648,897 Great Britain Jan. 17, 1951 OTHER REFERENCES Sugarman, The American Pressman, November 1955,, pages 35-36.

Remy, Treatise on Inorganic Chemistry, vol. II, Elsevier (1956), pages 440-441.

Meyers, Evaluation of a Xerographic Process for Prepa ring Zinc Oxide-Silicone, Binder-Type Lithographic 20 Plates, US. Army Engineer Research and Development Laboratories, Fort Belvoir, Va, Report 1545-TR, Project 835-O9-500 (8-35-09-112), Unclassified Report, pages l, 2, 13, 19 and 20 (Sept. 23, 1957). 

1. A METHOD OF PRODUCING LITHOGRAPHIC PRINTING PLATES, WHICH COMPRISES, CHARGING A SUPPORT HAVING A PHOTO-CONDUCTIVE SURFACE CONSISTING ESSENTIALLY OF PHOTO-CONDUCTIVE ZINC OXIDE IMBEDDED IN AN ELECTRICALLY-INSULATING RESINOUS FILM-FORMING LAYER COVERING THE FACE OF SAID SUPPORT TO PRODUCE AN ELECTROSTAIC CHARGE THEREON, EXPOSING THE CHARGED SUPPORT TO A LIGHT IMAGE OF MATERIAL TO BE COPIED TO PRODUCE A LATENT ELECTROSTATIC IMAGE OF SAID MATERIAL, APPLYING HYDROPHOBIC RESINOUS DEVELOPER POWDER TO THE LATENT ELECTROSTATIC IMAGE THUS PRODUCED SO THAT SAID HYDROPHOBIC RESINOUS POWDER ADHERES ELECTROSATICALLY TO THE CHARGED IMAGE AND NOT TO THE BACKGROUND AREAS OF SAID PHOTO-CONDUCTIVE SURFACE, FIXING THE HYDROPHOBIC RESINOUS POWDER TO PRODUCE AN ELECTROSTATIC COPY IN WHICH THE HYDROPHOBIC RESINOUS LAYER THUS FORMED DEFINES THE IMAGE AREAS AND THE SAID ELECTRICALLY-INSULATING RESINOUS LAYER HAVING THE ZINC OXIDE IMBEDDED THEREIN CONSTITUTES THE BACKGROUND AREAS, AND SWABBING THE THUS PRODUCED ELECTROSTATIC COPY WITH AN AQUEOUS SOLUTION OF A SALT FROM THE GROUP CONSISTNG OF THE HALIDES AND SULFATES TO ALUMINUM AND FERRIC IRON TO REACT THE ZINC OXIDE IN THE SAID BACKGROUND AREAS WITH THE SAID SALT AND THUS PRODUCE A SOLID HYDROPHILIC FILM CONSTITUTED SUBSTANTIALLY ENTIRELY OF THE REACTION PRODUCT OF SAID SALT AND THE ZINE OXIDE IN THE BACKGROUND AREAS, WHICH FILM COVERS THE BACKGROUND AREAS, RESISTS THE PENETRATION OF LITHOGRAPHIC FOUNTAIN SOLUTIONS AND REPELS LITHOGRAPHIC INKS. 